This application claims the priority of German application 196 25 503.1-21 filed on Jun. 26, 1996, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a steering apparatus for motor vehicles of the type with a nonmechanical coupling of a steering angle setting device and a steering angle input device.
Compared with conventional steering apparatuses with mechanical coupling between the steering-angle input device and the steering-angle setting device, the steering apparatuses with nonmechanical coupling between these two devices lack direct mechanical transmission of a reaction torque from the steering-angle setting device to the steering-angle input device. In order to maintain this feedback information, with which the driver of the vehicle is familiar, for steering apparatuses with nonmechanical coupling between the steering-angle input device and the steering-angle setting device, it is known practice to couple to the steering shaft a device which produces a reaction torque. Thus, German Patent Document DE-A1 42 41 849 discloses a steering system for vehicles or ships with a steering wheel having an associated steering shaft and a steering-angle setting device coupled electrically therewith, in which the steering shaft can be subjected by means of a torque-loading unit to a reaction torque that can be set as a function of the speed and/or operator's intention.
In the earlier German Patent Application 195 39 101.2, which is not a prior publication, a description is given of a steering apparatus of the type stated at the outset in which the device which produces a reaction torque comprises a return-spring arrangement, on the one hand, and a motor coupled to the steering shaft, on the other. In one exemplary embodiment, the return-spring arrangement comprises two helical compression springs arranged transversely to the steering shaft and a driver plate, arranged between them, which is coupled to the steering shaft by a mechanism. In another example, a piston-cylinder unit coaxial to the steering shaft is provided and its U-shaped piston is pressed by a helical compression spring surrounding the steering shaft against a slotted plate seated on the steering shaft in a manner fixed against rotation. In its opposite end region, the piston delimits, together with the cylinder, a working space which can be filled with a pressure fluid.
A technical problem underlying the invention is to provide a novel steering apparatus of the type stated at the outset which is optimized as regards the production of a reaction torque and compact construction.
The invention solves this problem by providing a steering apparatus having a mechanism which couples the device which produces a reaction torque mechanically to the steering shaft, which mechanism contains a steering-shaft pinion and a rack which is coupled to the latter and serves as a piston rod for at least one piston, which is guided in axially moveable fashion in a cylinder and, together with the piston rod, forms the driver unit by means of which the element that can be deflected elastically in a linear manner is coupled to the mechanism. This design of the mechanism permits compact construction of the system comprising the steering-angle input device and the device which produces a reaction torque mechanically, and permits the setting of any desired transmission ratio. In motor vehicles with an adjustable steering wheel, the compact construction of this system has the advantage that it does not restrict to any significant extent the foot space as a pivotally mounted steering shaft is pivoted. Moreover, this mechanism allows the setting of a rising reaction torque even in the case of steering angles which go beyond 360.degree..
In the case of certain preferred embodiments, the steering apparatus is developed such that the device which produces a reaction torque mechanically is positioned in a compact manner. Symmetrically, transverse to the steering shaft, a rack serving as a common piston rod for two pistons situated opposite one another is provided, of which one piston is in all cases taken along by the movement of the rack, depending on the steering direction, i.e. depending on whether the steering shaft has been deflected in one direction of rotation or the other out of its central position.
In certain preferred embodiments, the steering apparatus is developed such that the device which produces a reaction torque mechanically contains two pistons which are accommodated in a cylinder and against each of which the helical compression spring as the element that can be deflected elastically in a linear manner is supported. In a further development in this context, the cylinder is arranged next to the steering shaft with its longitudinal axis parallel to the steering shaft, this arrangement advantageously limiting the installation space required transverse to the steering shaft.
In the case of a further development of preferred embodiments of a steering apparatus according to the invention, a respective spring stop element is provided as a coupling element between the rack and the piston, and this spring stop element can replace a torsionally elastic coupling between a steering wheel and a steering shaft and contribute to noise damping.
In the case of certain preferred embodiments of the steering apparatus, the mechanism contains an eccentric disc which is coupled rotatably to the steering shaft and against the outer circumference of which there rests one end of a piston rod of a piston which is guided in axially moveable fashion in a cylinder and, together with the piston rod, forms the driver unit for coupling to the mechanism the element--of which there is at least one--which can be deflected elastically in a linear manner. The desired transmission characteristic between the rotary motion of the steering shaft and the linear motion of the element that can be deflected elastically in a linear manner can be set by a suitable choice of the shape of the eccentric disc and of the transmission ratio of the mechanism.
In the case of a steering apparatus developed in accordance with certain preferred embodiments, the piston delimits in the cylinder at least one working space, which can be filled with a pressure fluid. By appropriate pressurization with pressure fluid, a pressure force which is added to or subtracted from the elastic force of the element that can be deflected elastically in a linear manner can be produced in order to set a desired reaction-torque characteristic.
In the case of certain preferred embodiments of a steering apparatus, a device which produces a reaction torque electrically and has a motor is provided in addition and, specifically, is designed in such a way that the total reaction torque produced counteracts the respective change in the steering angle in a constant manner in a low speed range and counteracts a respective increase in the steering angle in a constant manner in a high speed range. This has the effect that, at low speeds of travel, the driver has to exert a steering torque of approximately the same magnitude when steering back in the direction of the straight-ahead position as when turning the steering by a relatively large amount, while, in the high speed range, automatic return of the steering to the straight-ahead position is assisted.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.